You may have mentioned it, from context if I missed that I did settle on more a large sniper size myself.
As big as a hummer nice, liking these canons a lot more, that is how I also imagined them when they destroy the astroids that and after googling a bit I settled on a modified version of this

The internal lasers are in a circular configuration, with 2 concentric circles. Not sure if I mentioned that. So the main structure would be circular, not rectangular.
They are then aimed with a holographic crystal, which has a reconfigurable internal structure that works similar to how a DLP projector works. Basically, it has tiny mirrors that can be adjusted by the second to create what looks like a 3D structure to the naked eye. This would give it more of a wide bore look, almost like a jet engine.
Because they repurposed the technology to use in a cannon, it can actually envelop an object, and create far more damage, vaporizing something far larger than would be possible with a cannon of a similar size.
Due to it being a sort of holographic laser projection, it can also target multiple points with near surgical precision, within an area of fire. So, it could do something like cut a cannon off of a ship or cut multiple docking clamps at once, if they were close enough together, or it was far enough away, for example.
I wanted it to be something special.
Thank you for that, really cool to know how you imagined it , very understandable that writing a story can take a lot out of you especially when you actually imagine things in such detail but have to decide how to plainly write it down.